When I call the captions.download endpoint with an ID that we retrieve from the captions.list endpoint, it always returns a 403. For example:
what you wrote above about "only works for videos your google account owns" is not my experience. I just successfully ran captions.download on a video (about dog training) which I definitely do not own - do not even have a dog. However, I have tested the exact same code on the video mentioned here on stackoverflow and get a 403 error.
So no it doesn't always return a 403 sometimes it returns a 200! Try it with the dog video mentioned above:
python captions.py --videoid="=jBN2_YuTclU" --action="download" --captionid='8S2GjnNfitU5HHoLyTeLxq_W1dP29YRFC8E8vFBUtws='
with the code you probably already have here.
It will need your client_secrets.json downloaded from the Google credentials page somewhere and a missing file youtube-v3-api-captions.json which you can get from here. The code launches a browser where you log in for OAuth2 authorisation.
Still, there must be a reason why it works for some videos and not others. @Abhishek might have it above. The wrong comment has been upvoted there. Nothing in the output of captions.list for a video that allows captions downloads and a video that does not is obviously different which would explains why one works and the other does not. If anyone can supply which {'key':'value'} pair in the youtube api controls this would be helpful.